rpavlis wrote:As a retired University chemistry professor, I hope it is in my power to provide some help understanding some problems that arise using machines such as the Europiccola and the like.
Around 1800 a man by the name of John Dalton stated his law of partial pressures. It says that when gases are mixed, the total pressure is equal to the sum of the partial pressures of each gas. Another principle of gas behaviour is the ideal gas law, PV=nRT. Modern students seem to call it "puvnert". P is pressure, V volume, n the number of moles, R a constant, and T the temperature in Kelvins. Still another is the principle of vapour pressure. Molecules in liquids and solids are held together by intermolecular forces of widely differing strength depending on molecular structure. Kelvin temperature is a linear measure of average molecular translational energy. Vapour pressure results from molecules in liquids (and sometimes solids) having sufficient energy to escape the intermolecular forces.
When we turn start a lever espresso machine it is a closed system. The space above the liquid is filled with air and a small amount of water vapour. Atmospheric pressure is usually around 100 kpa or 1 bar. The vapour pressure of water at room temperature is about 3 kpa. Inside the reservoir the air pressure is about 97 kpa and the water pressure is the vapour pressure, 3 kpa.
When we turn on the heat the temperature rises. The partial pressure of the air will raise by PV=nRT, and the water pressure will rise as its vapour pressure rises. Some of the air, however, is forced by the rising pressure into the upper part of the group above the piston which remains fairly cool because this air does not exchange with the reservoir. On machines with pressurestats the heating element is shut off when the total pressure inside reaches about 200 kpa. The air pressure at this point is a bit over 100 kpa and the water vapour pressure is a bit under. This results in a boiler temperature of around 100C. The group is not heated much, because the water vapour diffuses very slowly through the small passage to it. Some call the air pressure false pressure. Actually there is NOTHING false about this air pressure! We must get rid of this air, so we open the steam valve. Most of the air is quickly swept out.
If we touch the group at this point it is only slightly warm. While the air is being released we also need to raise the group lever about 3/4 of the way a couple of times to purge it of air too. It almost instantly becomes hot to the touch, hot enough to burn one's fingers. With the air gone the only gas in the system is now water vapour. The pressurestat turns off when the water vapour pressure reaches about 200kpa, around 120C.
Now we raise the lever and allow a few mL of water to pass through. The top of the group is already at 120 degrees, kept there by steam constantly condensing to keep it there. This heats the bottom. Now we put our filter basket in the portafilter and raise the handle. Air rushes through the porous coffee into the space below the portafilter. When we get the handle clear to the top the opening to the water inlet is exposed and very hot water enters inside the piston skirt. Some of it vaporises and mixes with the air--Mr. Dalton is at it again. There is a burst of pressure which forces much of the air downward through the coffee again, along with a lot of water vapour. As long as the coffee is not flooded with liquid water gases pass through it, and air is lost from the system. Once the coffee floods water is so viscous that the remaining air is trapped. If the piston skirt or cylinder wall be too cold not enough air will leave before this occurs and the pull will be spongy.
When we pull the handle down the pressure increases to about 1000 kpa. The vapour pressure of water is about 90 kpa at the 95 or so degrees of the waer, so 910 kPa is air. Gases are much more soluble at high pressure. Some of this high pressure air dissolves in the water. The high pressure also forces the water through the coffee. As it passes through the dissolved air forms bubbles as the pressure falls and air solubility decreases. This air interacting with surface active materials in the coffee form the foam which we call crema!
What?? Cripes man! Here am I thinking making a cappa outa the lever in the morning was one of life's simple pleasures. Now every time I look at the damn thing my eyes will bleed .